The invention relates to a power supply apparatus for electronic circuits operated at high-voltage potential. More particularly, the invention relates to such an apparatus which supplies power to such circuits independently of the electrical conditions on the high-voltage end.
Supplying electronic circuits at high potential with power presents considerable difficulties because a large potential difference must usually be bridged to supply operational power from the low potential side.
If the potential difference is in the order of up to about 110 kV, current transformers can be used at a cost which is still justifiable to assure the supply of power to electronic circuits arranged at high-voltage potential. For potential differences above 110 kV, the use of current transformers is no longer advisable for physical and cost reasons.
It is known from U.S. Pat. No. 52,477 of the German Democratic Republic to use power sources such as storage batteries or diesel engine units for supplying power to electronic circuits at high-voltage potential; these power sources were arranged at high-voltage potential. However, this type of power supply has not found acceptance for reasons of reliability and maintenance.
According to this reference an insulating mechanical shaft or an insulating belt has been used for transmitting energy from the low-voltage to the high-voltage side. The belt or shaft was moved by a motor located at low-voltage potential and a generator located at high potential is driven by the shaft or the belt and generates the current required for the power supply. These known power supplies are relatively expensive and not very reliable.
In view of the foregoing and as disclosed in the German patent, a pressure medium, particularly compressed air, has been used as the energy source at ground potential. The pressure medium activates a pressure-medium drive of a generator delivering a high-voltage potential. The compressed air is fed through insulating tubes or insulating hoses. This power supply too is very costly and also not very reliable because of the many individual parts required. Such a power supply is unsuited for devices which serve, for example for obtaining measured quantities needed for system protection equipment.
The publication Electronics of May 17, 1965, page 73 describes using a saturated current transformer for supplying current to electronic circuits at high-voltage potential; this assures an adequate power supply under normal operating conditions. In addition, it presupposes that at the high-voltage potential, for example in a high-voltage conductor, a sufficiently large current flows. If the voltage becomes very small or fails altogether, power can no longer be supplied by means of the saturated current transformer. Electronic circuits at high-voltage potential which measure, for example, the current flowing there, will then no longer furnish measured values. From the absence of a measured value it should not, however, be concluded that the current value is zero, because this may also occur with small currents in the event of a power supply failure. Connected protective devices therefore no longer receive an unambiguous signal in that case, so that it is possible that a section of line which should be disconnected, is not disconnected, or an unnecessary disconnect action is carried out.
According to the above-mentioned edition of Electronics, the current supply equipment is provided additionally with a current path from the high-voltage side to the low-voltage side. The current path consists of a series connection of a winding of a transformer and a voltage divider. A current sufficient to supply power to the electronic circuits at high-voltage potential is taken from the hhgh-voltage line through the transformer. The arrangement of the series circuit consisting of the transformer and the voltage divider is not advantageous, particularly because the voltage divider is of relatively complicated configuration and is trouble-prone. Furthermore, the power consumption is relatively high and excessive temperature develops in the voltage divider. A particular disadvantage of this known arrangement is that a failure of the high voltage is accompanied by a failure of the power supply for the electronic circuits.
A further disadvantage of the known arrangement is that the current through the current path is no longer proportional to the conductor voltage because of the non-linear voltage drop at the power supply.